1. Field Of The Invention
The present invention relates generally to carbonaceous fibers for use in friction materials and methods of preparation therefor. More particularly, there is provided a friction material and method of manufacturing therefor having improved mechanical properties which are useful in the nuclear, aerospace, aircraft and industrial fields. These composites have found particular acceptance as brake pads for aircraft, railroad and racing cars.
2. Description Of The Prior Art
The interest in friction materials having non-asbestos carbon/carbon composites has rapidly increased over the past decade. These composites retain their strength, modules, and mechanical properties up to higher temperatures than other materials. Generally, phenolic resins reinforced with carbon fibers have been used as starting materials for the preparation of carbon/carbon composites. The carbonization of these composites results in shrinkage and formation of cracks in the ultimate carbon matrix. The interaction between matrix and fiber may determine whether the resultant carbon/carbon composite will behave as a brittle, flaw sensitive material, or as a tough, thermal stress resistant composite. Therefore, the properties of the matrix as well as the fiber/matrix bonding are important factors in determining the fracture behavior and final mechanical properties of the composites.
The primary cause of disc failure in multi-disc brakes that are used in aircraft and railroad cars is thermal stress caused by thermal gradients. High heat input caused by the high aircraft deceleration requirements at high speeds and under great loads results in thermal stress which often causes failure of the brakes by dishing or cracking.
Friction elements such as automotive brake linings and disc pads, railway brake blocks and clutch facings can be made by bonding a friction material with a resin. Asbestos has been used as a friction material, either as a woven fabric or in fiber form. The asbestos is often bonded with a phenolic resin. When such asbestos type friction elements are subjected to severe brake or clutch operating conditions, the high temperatures produced tend to breakdown the element with resulting wear and/or loss in frictional properties.
In spite of this, friction elements made hitherto from material other than asbestos have failed to match the performance of asbestos elements. Asbestos has conventionally been used as principal reinforcements for friction materials after being shaped and cured together with thermosetting resins and auxiliary reinforcements. Asbestos, however, is a carcinogenic substance which presents safety and health problems during both manufacture and use. Under these circumstances, demand has increased for non-asbestos friction materials. Friction materials generally are required to have friction coefficients of from 0.3 to 0.5 to ensure high wear resistance and mechanical strength at temperatures of about 500.degree. C.
Conventional friction materials molded from a mixture of asbestos fibers, thermosetting resins and/or vulcanizable elastomers, particulate fillers and friction modifiers are not normally consolidated to achieve the theoretical densities of the mixtures. These mixtures usually contain proportions of voids, even when cured at temperatures of up to 160.degree. C. and pressures of up to 3 tons per square inch. The presence of voids, especially when interconnected to produce a permeable structure, is desirable to produce materials with good resistance to "fade". That is, a reduction in the coefficient of friction occurs when the friction materials operate at high temperatures.
In recent years, carbon fibers and aramid fibers have come to be preferred over asbestos as fiber materials for use in friction materials such as automotive disk brake pads, clutch facings and the like. This is because the carbon and aramid fibers have superior anti-fade properties when compared to asbestos, while maintaining favorable wear properties.
U.S. Pat. No. 3,650,357 to Nelson et al, which is herein incorporated by reference, discloses the making of an aircraft disc brake in which each disc of the brake stack is made from a carbon-based material having high specific heat, low density, low thermal expansion properties and good thermal stability over a wide range of load characteristics.
U.S. Pat. No. 4,490,201 to Leeds, which is herein incorporated by reference, discloses a method of making a carbon composite wherein a fabric comprised of oxidized or stabilized polyacrylonitrile (PAN) is heat treated to totally carbonize the fabric and then coated with a resin. The carbon fabric is then further heat treated to thermally fuse the resin to the fabric.
U.S. Pat. No. 5,051,300 to Rousseau discloses a carbon/carbon composite for use as a high performance heat protector which contains an outer coating of SiC.
A variety of fibrous materials have been used as non-asbestos materials. For example, the known art uses preoxidized fibers produced by heat-treating acrylic polyacrylonitrile (hereinafter "PAN") fibers at from about 200.degree.to 400.degree. C. in air. However, the preoxidized fibers with a tensile strength of from about 1 to 3 g/d and a tensile modules of elasticity of from about 50 to 150 g/d are not satisfactory as fibrous reinforcement materials. In particular, the fibers are appreciably low in tensile modulus of elasticity as compared with asbestos. In addition, the weight of the fibers reduces at about 500.degree. C., which is the temperature that should be withstood by friction materials. Furthermore, the fibers have a tendency to form cracks in the surface of the friction material due to effluent gas and shrinking in volume of the fibers.
The use of carbon fibers derived from PAN as fibrous reinforcement materials has also been disclosed, for example, in U.S. Pat. No. 4,259,397. The carbon fibers are usually produced by calcining oxidized fibers at 1000.degree. C. or higher in an inert gas to obtain fibers having a carbon content of at least 90 wt % and a bond nitrogen content of from about 1 to 8 wt %. These fiber types typically exhibit a tensile strength of at least about 13 g/d, a tensile modulus of elasticity of from about 1,400 to 1,800 g/d and an electric specific resistivity of no more than about 0.01 ohm-cm. However, when they are used as reinforcement for friction materials, the carbon fibers provide low friction coefficients, properties that are disadvantageous to braking and power transmission. Moreover, the carbon fibers have the further disadvantage that their high heat conductivity causes increased heat transfer to the support of a friction material or the mating member (usually made of a metal), thereby impairing the mechanical characteristics of the mating member. Generally, PAN based carbon fibers contribute to the stabilization of the coefficient of friction of the friction material at high temperatures.
The following U.S. Patents are also typical of the prior art and are all incorporated herein by reference:
U.S. Pat. No. 5,004,497 to Shibata et al discloses a friction material containing 0.85 to 30% by weight of carbon fibers and 2 to 20% by weight of aramid fibers. U.S. Pat. No. 4,861,809 to Ogawa et al teaches a friction material containing from about 0.5 to 29% weight of carbonaceous fibers with a bond nitrogen content of from about 14 to 21% weight, which have been made carbonaceous while under tension, an auxiliary material and a thermosetting resin. Carbonaceous fibers formed under tension, however, have voids and cracks thereby degrading their mechanical strengths. U.S. Pat. No. 5,292,780 to Godfrey et al describes a friction material and a binder, wherein the binder is a blend of polyvinyl alkyl ether and a phenolic resin. Optionally, the binder could further include a rubber. U.S. Pat. No. 4,656,203 to Parker describes a non-asbestos friction material having reinforced fibers embedded in a matrix of binder material.
Although the foregoing references exemplify the advancements which have been made in the art of friction materials, there nonetheless exists a long felt need to provide an improved non-asbestos friction material. The present invention is concerned with improved non-asbestos carbonaceous resin composites suitable for the preparation of friction elements, and friction elements prepared therefrom. These friction elements may be employed in the form of a fiber, a paper, a spun yarn or the like.
It is therefore an object of the instant invention to provide improvements in the art of friction materials which employ non-asbestos materials.
It is a further object of the instant invention to provide a friction material having low thermal conductivity, high thermal stability, high electrical resistivity, high bond strength and improved coefficient of friction properties.
We have found that these and other objects of the instant invention can be attained using oxidized polyacrylonitrile based carbon fibers as described below.